1. Field of the Invention
This invention relates to a driving method for an AC driven capacitive flat matrix display panel, that is, a thin film EL display device, and driving circuit thereof.
2. Description of the Related Art
Conventionally, the thin film EL display device has been driven by a field reversal drive unit which was equipped with a N-ch MOS driver and a P-ch MOS driver as scanning side electrode drive circuits. The system reverses the polarity for each field (for each line sequential drive of a field).
In the U.S. patent application Ser. No. 737,068, now abandoned in favor of a continuation application, Ser. No. 229,751, now allowed, filed on May 23, 1985 (the counterpart in West Germany is Application No. P3518596.1 filed on MAY 23, 1985), the applicant has proposed a driving unit in which luminous intensity irregularity caused by the polarity inversion of the voltage applied to the panel can be eliminated and flickers in a picture can be minimized by reversing the polarity of write waveforms applied to picture elements of each scanning line.
Furthermore, in the U.S. patent application Ser. No. 894,509, now abandoned in favor of a continuation application, filed on May 19, 1986 (the counterpart in West Germany is Application No. P3619366.6 filed on June 9, 1986), the applicant has proposed a drive circuit in which a burning phenomenon of the EL layer resulting from polarization is avoided, the service life of the display panel can be lengthened, and the power consumption can be reduced. This is achieved by creating pulse voltage waveforms with positive and negative which are applied to the picture elements of the EL display panel is a perfectly symmertrical manner by the use of a push-pull type of driver IC provided on the data side.
However, in these driving methods and driving circuits, assuming that the brightness at the time of lighting of all picture elements in an EL panel having 640 picture elements in a frame is 100, as shown in FIG. 8(a), the brightness becomes substantially 110% when 100 picture elements (1/6 picture elements) are lighted. This occurs even though this value is changed in accordance with the capacity of driver ICs used for the EL panel, width of write pulses, write voltage, or the capacity of the EL panel.
In these drives, if strip patterns 32a to 32c, as shown in FIG. 8(b), negatively are displayed on an EL panel 10, the brightness of areas 33a to 33d in which all bits are lighted becomes lower than that of area 34, 35, and 36 in which the number of bits which are lighted is smaller than the former. More importantly, the difference in the brightness between the area 33d and the area 34 becomes significantly great. As a result of this, even in the emitting area (white region shown in this figure), strip patterns 34 to 36 become distinctively visible.
The reason for this is due to the fact that since loads of the lines of the EL panel are different due to the difference in the number of light emitting picture elements in the lines panel, voltage waveforms applied to the lines by EL driving high withstanding ICs are different. In FIG. 8(c), reference numeral 41 represents a voltage waveform applied to a line in which there are a relatively larger number of light emitting picture elements, reference numeral 42 represents a voltage waveform applied to a line in which there are a relatively smaller number of light emitting picture elements. Reference numerals 43 and 44 represent current waveforms corresponding to the voltage waveforms 41 and 42 respectively. The inclinations of the waveforms of the voltages 41 and 42 are the same until the voltage reaches Vth at which the panel starts emitting light. However, if the voltage exceeds Vth, the current is made constant due to the characteristics of an EL driving withstanding IC, and the inclination of the voltage waveform is made moderate. This occurs despite the fact that the greater current flows to the line in which there are a larger number of emitting picture elements. Therefore the time taken for the voltage to be applied to the picture elements, from the final voltage V.sub.W to the starting voltage of discharge, becomes tB&gt;tA. Consequently, the brightness difference occurs. Symbol tw represents the period of time over which the voltage is applied.
As described above, since the emitted brightness of the display panels is varied due to the number of the emitting picture elements of a scanning electrode, a problem arises in that the quality of the display deteriorates.